1. Field of the Invention
The present invention is concerned with a network for compensating the frequency response of a spectrum analyzer, in which the heterodyne oscillator off the network, which cooperates with a mixer in the frequency synthesizer circuit, is controlled by way of a sweep voltage of a sweep generator, the network comprising a controllable level varying circuit disposed in the frequency synthesizer circuit for controlling the output level in response to the sweep voltage in a frequency-compensating sense.
2. Description of the Prior Art
For frequency-selective measurement off the level of a high frequency (Hf) voltage, spectrum analyzers are employed which operate on the heterodyne principle with a hyterodyne sweep-frequency oscillator and convert the Hf input signal to be measured either to an intermediate frequency or directly to a D.C. voltage signal. The frequency synthesizer off such a spectrum analyzer includes various mixers with filters, frequently also level dividers and other components, which cause an undesirable frequency response, i.e. an undesirable variation of the level in response to the respective frequency. In addition to such an undesirable frequency response, the spectrum analyzers also, in most cases, have an undesirable temperature dependents.
In order to eliminate such an undesirable frequency response and in order to achieve an accurate level measurement, it has been known in the art (Spectrum Analyzer hp 8568A of Hewlet Packard) to derive a frequency-proportional analog control value through an additional resistor-diode network to which the sweep voltage is supplied, the control value being used to change the level in the spectrum analyzer in a frequency response compensating sense. To this end, the frequency synthesizer of the spectrum analyzer has a respective controlled amplifier or controlled attenuator provided, preferably in the intermediate frequency (If) circuit or in the D.C. voltage circuit after a demogilator. Such a known compensating analog network can only eliminate the average frequency response at a reasonable circuit design effort, but is unable to eliminate it completely.
In processor-controlled measurement receivers in which the heterodyne frequency can be adjusted by way of a synthesizer through the processor system, it is known per se for compensating the frequency response to provide an additional digital memory for storing the corresponding level-correcting values for the various heterodyne frequencies in the form of digital values, and for the compensation of frequency response, this stored digital value is then directly used in the processor system for correcting the measured value. Although this compensation method is as accurate as desired, it has the drawback that it cannot be used at a rapidly-varying frequency such as in the case of a spectrum analyzer.
"It is known with respect to spectrum analyzers to provide, in the frequency synthesizer circuit behind the mixer, a variable gained amplifier" (Zirwick, Eine Neuentwicklungauf dem Gebiet der Spektrum Analysatoren, Frequenz, Vol. 8, No. 3, 1974, pp 66-72) by way of which the output level is manually adjustable by the user. Furthermore, it is known in a microprocessor-controlled sweep generator to provide a controllable amplifier in the frequency synthesizer circuit for adjusting the modulation amplitude, the amplifier being likewise controllable through the microprocessor (Hewlett Packard Journal, September 1982, pp. 30-36.)